(1) Field of the Invention
The present invention relates to an electrically conductive adhesive sheet having an electrically conducting property across a thickness thereof, and an electrically insulating property along a direction of a surface thereof. The present invention further relates to a circuit board having such a electrically conductive adhesive sheet and an electrical connection structure using such an electrically conductive adhesive sheet.
(2) Prior Art of the Invention
Recently, electronic equipments and electronic parts have been made compact, and in association with this, it has been required to connect a plurality of conductive track patterns arranged on a circuit board, e.g., flexible circuit board or rigid circuit board at a fine pitch with another conductive track patterns arranged similarly on another flexible substrate for example at a fine pitch corresponding to the former conductive track patterns, or with a plurality of conductors such as lead terminals for IC, etc. arranged at a fine pitch. Such a connection as above is normally carried out by a so-called wire bonding with use of a gold wire. etc., or a soldering in association with dipping in solder, for example.
However, the wire bonding with use of the gold wire causes an awkward connection working and increase in cost because the gold wire itself is high in cost and collective connection of the wire is impossible. Further, there occurs a problem in reliability such that each of the wire is brought into contact owing to a fine pitch of a plurality of connection parts, or a connecting strength of bonding parts is reduced and the connection parts are short-circuited owing to reduction in width of the bonding pads.
In case of the dipping in solder, there similarly occurs a problem in reliability such that a solder is flown from the connection parts to cause an accident such as short-circuit between the connection parts due to a fine pitch thereof.
Further, there is another connecting method with use of a so-called zebra connector. According to the connecting method, insulators and conductors are alternately layered at a predetermined pitch to form a connector, which is in turn electrically interposed between conductors to be connected with each other and is mechanically pressed on both sides of the conductors to hold the connector. However, the zebra connector itself has no function to mechanically fix the conductors, and therefore it is difficult to maintain a predetermined width with a uniform pressure.
Further, there has been proposed another connection method wherein an adhesive layer prepared by alternately and stripedly coating a conductive hot-melt adhesive containing carbon fibers and an insulating hot-melt type adhesive is interposed between conductive track patterns and the corresponding conductors. However, reduction of a width of such a stripe as above is limited, and therefore this method is not suitable for connection of the conductive track patterns having a fine pitch. Further, it is awkward to manufacture such an adhesive layer.
Still another method is disclosed in Japanese Patent Publication 2223/72 published on Jan. 21, 1972. According to this method, wiring patterns are formed through a thermoplastic adhesive material on a substrate, and both substrates are bonded by heating under pressure with the molten adhesive material. However, there occurs a problem in reliability such that when the adhesive material is molten by heating, the wiring patterns are moved to cause a short-circuit between the patterns. Therefore, it is impossible to satisfactorily make the patterns with a fine pitch and a high density. At this time, as both the substrate are required to be bonded by deformation due to bending of the substrates, at least one of the substrates is required to be flexible. Additionally, there occurs a problem in reliability such that when a spacing of the patterns is small, sufficient bonding strength cannot be obtained, or in the case that patterns made of Cu foil are bonded to the substrate, a sufficient amount of adhesive material for filling a space between the patterns and bonding both the substrates cannot be obtained.
In yet further method, it is known to use a connecting sheet 1 as shown in FIG. 1. The connecting sheet 1 comprises a releasing sheet 5 and an adhesive layer 4 containing an electrically conductive fiber 3 such as carbon fiber arranged in one direction in an insulating adhesive material 2, which layer 4 is applied on the releasing sheet 5. The electrically conductive fiber 3 is mixed in an amount of 5-20 parts by volume, for example, with respect to 100 parts of volume of the adhesive material 2. A thickness of the sheet-like conductive layer 4 is set to 20-120 microns under unused condition prior to electrical connection of wirings as hereinafter described. A carbon fiber having a diameter of 5-50 microns and a length of 0.05-3 mm, for example, is used as the electrically conductive fiber.
As shown in FIG. 2 which is an enlarged plan view of FIG. 1 and as shown in FIG. 3 which is a cross section taken along the line A--A in FIG. 2, connection with use of the connecting sheet 1 is carried out by laying a plurality of conductive track patterns on another conductors, and interposing the insulating adhesive layer 4 as peeled off from the releasing sheet 1. In this example where a plurality of conductive track patterns 6 attached and arranged on a rigid substrate 8 are connected to the corresponding conductors 7, e.g., conductive track patterns attached and arranged on a flexible substrate 9, both the substrates 8 and 9 are layered in such a manner that both the conductive track patterns are extended in the same direction at connection parts thereof and are almost registered with each other. Then, as shown in FIG. 4 which is a cross section taken along the line B--B in FIG. 2, the adhesive sheet 5 of the connecting sheet 1, that is, the adhesive layer 4 containing an electrically conductive fiber 3 dispersed in the adhesive material 2, is provided over the layered parts of the conductive track patterns 6 and the conductors 7 in such a manner that the conductive fiber 3 is extended along the conductive patterns and the conductors 7. Then, both the substrates 8 and 9 are bonded as shown by arrows (a) and (b) by heating under pressure application from outside thereof. Thus, as shown in FIG. 5, the conductive track patterns 6 and the corresponding conductors 7 are electrically connected with each other by the conductive fiber 3. Further, the conductive fibers 3 are electrically insulated from each other by the adhesive material 2, and accordingly if a diameter of the conductive fiber 3 is set to a value sufficiently smaller than a spacing between adjacent connected portions, electrical connection due to the conductive fibers 3 between the adjacent connected portions may be avoided, and simultaneously both the substrates 8 and 9 may be mechanically strongly bonded with each other by the adhesive material 2.
According to this example, as the conductive track patterns and the corresponding conductors are electrically connected anisotropically in dependence upon direction of the conductive fibers 3, a shortcircuit between the connected portions may be avoided to improve reliability. In addition, as a plurality of conductive track patterns and the corresponding conductors are simultaneously connected with each other, it is superior in mass production. However, because the electrical connection is carried out through carbon which has a relatively large resistance, there will sometimes occur a problem with respect to high resistance when current flows through the connected portions, though it is not a big problem when mainly voltage is applied to the connected portions with small current therethrough.
Such an adhesive material containing the conductive fibers or a connecting method using the adhesive sheet as mentioned above is disclosed in Japanese Published Unexamined patent application No. 138881/81 published Oct. 29, 1981 or European patent application Publication No. 0068739 published May 1, 1983.